Light modulation devices have many applications in photonics (telecommunication, imaging, energy conservation, etc.). The modulation may be activated by means of various mechanisms based on: mechanical movement, deformation, photochromism, charged particle motion, electro optic modulation in interferential or polarimetric schemes and finally by using electrically controllable light transmission.
The last approach is particularly interesting for shutter (imaging), energy saving (so called “smart windows”), privacy (image destroying) and color control applications. In addition, electrically controllable systems that are operated without polarizers are gaining in cost reduction, energy efficiency and reliability.
One of the traditional methods of obtaining electro optic modulation of light transmission is based on the use of Polymer Dispersed Liquid Crystals (PDLCs), as described in Doane, Chien, Yang and Bos chapters 1, 4, 5, 11, 12, 13 of “Liquid Crystals in Complex Geometries”, edited by GP Crawford & S. Zumer (Taylor & Frances, London. 1996). With reference to FIGS. 1A and 1B, such materials are typically composed from 25% of liquid crystal dispersed (in the form of droplets) into a solid polymer matrix (75%). While being efficient for privacy window applications, there are however several drawbacks with this approach: most important of them being that the light scattering provided is dominantly forward scattering and thus is not very efficient for energy flux control. Another drawback is the presence of the polymer matrix of the PDLC which contributes to yellowing of such a modulated pane, when used for example as a window exposed to sunlight. The high operating voltages and the angular dependent scattering (haze) are other significant drawbacks.
Electric modulation of light was demonstrated also in so called Polymer Stabilized Liquid Crystal (PSLC) compounds by R. A. M. Hikmet in “Electrically Induced Light Scattering from Anisotropic Gels”, J. Appl. Phys. 68, pp. 4406, 1990, where the polymer content is significantly reduced, typically to 5%, while the remaining mass (95%) is composed of liquid crystal. FIG. 2 shows an example of such a structure with a polymer concentration gradient, going from almost 100% of liquid crystal (right bottom corner) to almost 100% of polymer (left top corner), T. Galstian, “Liquid Crystals, Polymers, and Electrically Tunable Optical Components”, 19 Apr. 2010, SPIE Newsroom.
The light scattering may be controlled by the appropriate choice of material parameters. For example, in L. Komitov, L-C. Chien, S. H. Kim, “Method of Fabricating Electro-Optical Devices with Polymer Stabilized Liquid Crystal”, U.S. Pat. No. 8,081,272, Dec. 20, 2011 and M. Mitov, N. Dessaud, “Cholesteric Liquid Crystalline Materials Reflecting more than 50% of Unpolarized Incident Light Intensity”, Liq. Cryst. 34, no. 2, pp. 183-193, 2007, cholesteric (or “helical”) liquid crystal material was used in the above mentioned PSLC configuration to obtain preferential back scattering of light. While the back scattering is increased compared to the use of simple, so called “nematic”, liquid crystals, the main problem of photo chemical stability (yellowing in sunlight) remains. However, it is difficult to eliminate the polymer content since its presence is an important factor particularly for obtaining modulators of high efficiency.
Natural light may be presented as the sum of two orthogonal polarizations (two crossed linear polarizations or two opposed circular polarizations). The use of cholesteric liquid crystal material typically ensures the reflection (or back scattering) of only (mainly) one circular polarization, while the opposed circular polarization is not affected by the material. That is why, various “polymer matrix programming” methods have been developed to provide the reflection of both types of circular polarizations, see M. Mitov, N. Dessaud, “Cholesteric Liquid Crystalline Materials Reflecting more than 50% of Unpolarized Incident Light Intensity”, Liq. Cryst. 34, no. 2, pp. 183-193, 2007.
Further efforts were devoted by J.-P. Bédard-Arcand, T. Galstian in, “Self Organization of Liquid-Crystal and Reactive-Mesogen into 2D Surface Stabilized Structures,” Macromolecules, 44, 344-348, 2011, to the development of light modulators with less volumetric polymer content, by creating so called Surface Polymer Stabilized Liquid Crystal (S-PSLC) material systems. However, the complexity of manufacturing: mixing the liquid crystal with a polymerizable monomer composition, its handling, dispersion, polymerization, stability, etc. still remain a problem.
In some applications, such as for windows, providing a controllable reflection of 50% of incident light is practical to reduce the amount of light entering a window, even if control over substantially 100% of the light could be preferable. Being able to switch from reflection to transmission (either with diffusion or with transparency, or both), is desirable. Cholesteric Liquid Crystal (CLC) materials provide the ability to reflect light, however, the electric field strength required to change the state of the liquid crystal to remove the reflection can be nearly prohibitive.